Pressure swing adsorption is a well-known method for the separation of bulk gas mixtures and for the purification of gas streams containing low concentrations of undesirable components. The method has been developed and adapted for a wide range of feed gases, operating conditions, product purity, and product recovery. Many pressure swing adsorption systems utilize two or more adsorbent beds operated in a cyclic sequence in order to maintain a constant product flow rate while selected beds undergo various steps including adsorption, depressurization, evacuation, purge, pressure equalization, repressurization, and other related steps. Multiple adsorbent beds using numerous process steps are required to achieve high purity and/or recovery of valuable gaseous products such as hydrogen, carbon oxides, synthesis gas, light hydrocarbons, and the like. Multiple adsorbent beds using these process steps also are used to recover oxygen from air for various applications including portable medical oxygen concentrators.
Many of these pressure swing adsorption processes operate partially at pressures below atmospheric and are described in the art as vacuum swing adsorption (VSA) or pressure-vacuum swing adsorption (PVSA) processes. In the present specification, pressure swing adsorption (PSA) is used as a generic term to describe all types of cyclic adsorption systems regardless of operating pressure levels.
In pressure swing adsorption process cycles, the gas needed for the purge and repressurization steps is provided by gas obtained during other process steps. Repressurization can be accomplished by using final product gas, intermediate gas obtained by pressure equalization among beds, feed gas, or combinations thereof. Purge can be provided by intermediate depressurization gas from other beds and/or by final product gas.
Feed repressurization is disclosed in representative U.S. Pat. Nos. 4,406,675 and 5,540,758, and in European Patent Publication No. 0 354 259. The use of product gas for purge and/or repressurization is presented in representative U.S. Pat. Nos. 5,328,503, 5,411,578, 5,429,666, and 5,656,067. U.S. Pat. Nos. 5,330,561 and 5,203,888 disclose bed repressurization using pressurized feed gas or product gas.
It is desirable in pressure swing adsorption processes to minimize the amount of product gas used for purge and repressurization in order to increase product recovery. This can be difficult in many cases because of feed gas composition, product purity requirements, and product recovery requirements. In most pressure swing adsorption processes, for example, there is a tradeoff between product purity and product recovery wherein an increase product purity is accompanied by a decrease in product recovery. There is a need in the art for improved process cycles that allow increased product recovery at higher product purity levels, and this need is addressed by the invention described below and defined by the claims that follow.